Explaining the unimaginable: How do nuclear bombs work?

 

 So, this is a bit of a touchy question. From the beginning, even before the true destructive and killing power of The Bomb was clear to its own creators, one thing was certain: Nobody should ever acquire nuclear weapons if those who do have them can help it. That may be a hypocritical stance for current nuclear powers to take — but they’re live hypocrites, so whatever.
The justified fear over nuclear technology led to the creation of one of the weirder bits of legislation: the classification of “born secret.” This refers to the fact that, under a certain reading of the law, information on the design and function of nuclear weapons is considered stolen classified information even if you come up with it yourself.
As a result of this and more reasonable secrecy plans, the fine details of all but the earliest nuclear weapons are still unknown to the public. To be fair, those details wouldn’t help anyone but a state actor, one with access to nuclear material, enrichment facilities, tons of money, and at least a few world-class engineers, but it’s still probably for the best. So rather than learning how to actually build nuclear weapons (darn…), we’ll have to settle for a basic conceptual understanding of the two basic nuclear weapon types: the old, and the new.

Fat Man, the bomb which was detonated over Nagasaki. Shirts probably wouldn’t have helped anyway…

The old type of nuclear weapons, still in use all over the world, are called fission bombs, or simply atomic weapons. The latter term can be used as an umbrella, or to specifically refer to the first sort of bombs created during the Second World War. Scientists load the bomb with a “super-critical mass” of enriched fissile material, usually uranium, which can create a self-sustaining chain reaction of fission events. Though it’s created differently, this is basically the same chain reaction that occurs in a nuclear reactor, but more uncontrollably due to the amount and enrichment (isotope purity) of the sample used.
The ups and downs of this classical nuke-type are well known. The explosions are big, the fallout horrifying. Even hardened military men, who did not blink at the idea of wiping whole sections of world cities from existence, were taken aback by the continuing impact of radioactivity. This radioactivity comes from various sources, but much like the Chernobyl nuclear disaster, the majority of the long-term damage comes from how the explosion physically distributes radioactive material over a wide radius. This material comes from the bomb itself, and if detonated near the surface (not high in the air) the reaction will also vaporize and disperse a huge amount of ground and/or water as a highly radioactive (mushroom) cloud. Traces of the fallout from some such explosions can be detected by scientific instruments all over the world.

The other type of nuclear weapon is called a thermonuclear weapon, or sometimes a hydrogen bomb. While no nuclear weapon could ever be said to be humanitarian, if there ever is a nuclear war between great powers, we’ll be happy they have these, rather than the old kind. “Hydrogen bomb” gets the point across: The primary destructive sample is not uranium or plutonium or even thorium, but heavy isotopes of the most abundant element in the universe. And rather than breaking these atoms apart, thermonuclear weapons generate their outward force through the process of nuclear fusion.
Thermonuclear weapons basically contain a conventional nuclear weapon, but much smaller than its overall yield would seem to require. The power of this small fission reaction is directed onto two on-board samples of hydrogen isotopes — one deuterium, the other tritium — and these samples are forced together so violently that they fuse. Again, the main difference between the fusion going on here and in a fusion reactor (beyond that we had to explode a bomb to get it started) is that a bomb has many tens of thousands of times more fusion material.
Now, doing fusion also releases a ton of radiation, but this release is short-lived compared with the radioactive half-life of material strewn around after a fission explosion. So, by eliminating the vast majority of the fissile material in the bomb, distributing this sample around the environment via a fusion explosion has a much less toxic effect. Thermonuclear weapons can deliver a much higher yield than pure fission bombs, but they crucially produce less fallout while doing so. They are also more natural fits as so-called “tactical nukes,” allowing targeted destruction of an installation without having to make the whole area uninhabitable for a while.
Other types of nukes include neutron bombs, which intentionally let high-intensity neutron radiation out of the bomb’s case. While most bombs have a thick case of lead or some other material to stop the release of neutron radiation, neutron bombs are designed to be as thin and permeable as possible. They were specifically invented to counter the thick shielding on Soviet tanks, which was thought to provide too much protection against pure heat and concussion, but they’re also sometimes thought of as pure anti-personnel weapons, since neutron radiation can be extremely deadly to biological material without destroying infrastructure.
There’s also a concept called a salted bomb, which encases the nuclear device in a metal like gold that can be neutron-blasted into a much more radioactive isotope, producing a huge additional amount of radioactive material upon detonation. The concept was named for the phrase “salt the Earth;” thankfully, so far as we know no salted bomb has ever actually been tested.
None of these are to be confused with dirty bombs, which are the real threat posed when low-tech groups like terrorists come into nuclear material. Rather than creating an actual nuclear bomb, they would simply strap a regular explosive device to a sample of radioactive material and blow it up. This cannot cause a nuclear reaction, but it can contaminate large areas by distributing an aerosol version of the radioactive substance. The destructive power isn’t very great, but the loss of life could still be substantial due to health problems in the years and decades after the explosion.